This invention relates to a circuit arrangement and an electrical appliance with a buck converter and an inductive load as, for example, an electric motor, a relay or a solenoid valve. A buck converter is known to include a diode, a coil and an electronic switching device switching the current passing through the coil on and off. The output voltage of the buck converter may be higher or lower than a supply voltage provided, for example, by a battery or a power supply.
The art knows of electrical appliances in which a buck converter produces from a supply voltage a dc voltage used for operating an electric motor. These appliances may additionally include an electronic circuit which is equally supplied by the buck converter.
Furthermore electrical appliances are known which have an electric motor adapted to have its speed regulated or to be switched on and off by an electronic circuit including, for example, a pulse duration modulator and an electronic switching device. In particular in battery powered electrical appliances of this type it may happen that when the electric motor is turned on the supply voltage drops at least temporarily to such a low level that proper operation of the electronic circuit is no longer ensured, causing, for example, a microcontroller to be reset.
From DE 42 37 843 a circuit arrangement for operating an inductive load is known which comprises at least one first and one second part winding. The circuit arrangement comprises a control arrangement and is operated from a dc voltage source whose voltage is not sufficient for operating the control arrangement. The part windings of the inductive load are utilized as autotransformer in order to thus generate a higher voltage sufficient to operate the control arrangement.
It is an object of the present invention to make sure that an electrical appliance with an inductive load and an electronic circuit functions reliably also under a fluctuating supply voltage, and to provide a circuit arrangement suitable therefor.
This object is accomplished in particularly simple manner by a circuit arrangement in which, according to the present invention, the buck converter utilizes the inductive load as coil. In an electrical appliance of the invention the buck converter supplies an electronic circuit but not the inductive load.
The present invention will be explained by way of an example in which the inductive load is an electric motor. In this instance the circuit arrangement of the invention has the particular advantage of requiring very few components in order to implement a speed regulation for the electric motor on the one hand, and a buck converter for stabilizing the supply voltage for an electronic circuit on the other hand. According to the invention not only is the electric motor used as coil for the buck converter, but also the electronic switching device used for speed regulation is utilized as switching device for the buck converter. To enable the buck converter to operate also when the electric motor is at a standstill, it is necessary to maintain a minimum current flow, chopped by the electronic switching device, through the electric motor, that is, the load needing to be powered by the electric motor has to be of a magnitude causing the electric motor to stop at this minimum current. On the other hand, care must be taken to ensure that the electric motor is not at any time operated at full power, that is, with the current not chopped, because otherwise the buck converter does not operate either. The permissible pulse duty factor of the chopped current to ensure satisfactory operation of the electrical appliance depends on the dimension of the electric motor, the dimension of a storage capacitor charged by the buck converter, and on the current consumption of the electronic circuit supplied by the storage capacitor. The pulse duty factor may be in the range of, for example, 0.01 to 100, and where small electrical appliance are concerned rather in the range of 0.05 to 20 or 0.1 to 10.